In the industrial preparation of polyolefins, it is generally desirable to perform the reaction in a reactor kept at a determined temperature and at a constant production rate, in a continuous process, in order to obtain polyolefins having predetermined properties. In the preparation of polyolefins that is performed in the presence of a Ziegler catalyst, there are many properties of the produced polyolefins to be controlled. Of these properties, the melt index and the density are the most important factors. The melt index and the density of polyolefins reflect the average molecular weight and are optionally chosen, depending on the purpose for which the polyolefin is to be used. These indices enable consumers to choose the proper polyolefin for an intended use.
The melt index is usually measured at 190.degree. C. for polyethylenes and at 230.degree. C. for polypropylenes, according to the method of ASTM D1238. The polymerization rate of polyolefins is proportional to each of the concentrations of catalyst and monomer, independently, once the polymerization temperature and the catalyst are determined. In the polymerization in which a Ziegler catalyst is used, the melt index of the resulting polyolefin is controlled by hydrogen which acts as a chain transfer agent. Thus, the melt index of the resulting polyolefin is determined by the concentration ratio of hydrogen to olefin (other factors, including species of catalyst, temperature, volume and shape of the reactor also influence the melt index). The density of a polyolefin is measured according to the method of ASTM D1505-67. The density of a polyolefin is usually determined by various factors, including species of catalyst, temperature, volume of the reactor, amount of comonomer, and the like. Control of the density is usually performed by controlling the molar ratio of olefin to comonomer. Increase in the amount of comonomer generally results in decrease of the density. For the reasons mentioned above, preparation of polyolefins is generally carried out in a continuous process by feeding predetermined amounts of catalyst, olefin and hydrogen to a reactor kept at a predetermined temperature to produce a polyolefin having predetermined properties or meeting predetermined standards. Although it is required to feed predetermined amounts of catalyst, olefin and hydrogen to the reactor in order to maintain the reaction system under predetermined conditions, it is difficult to maintain the conditions merely by controlling the reactant feeds. For example, a subtle variation or decrease in the activity of catalyst in the polymerization reaction brings about an increase in the olefin concentration in the reactor, thus resulting in decrease of the melt index of the produced polyolefin. There are many indefinable factors that influence the product properties described above. The melt index of the resulting polyolefin is sensitive to variation of the ratio of olefin to hydrogen. In industrial processes for preparing polyethylenes, for example, the relationship between the change in the melt index (M.I.) and the change in the partial pressure ratio in gas phase is approximately as shown below, always depending on polymerization conditions such as species of Ziegler catalyst, temperature, and the like. ##EQU1##
In the case of polyethylene, a polyethylene product will be discarded as being below standard, if it has the melt index of above .+-.10% range out of the predetermined value. Therefore, .DELTA.(H.sub.2 /C.sub.2) needs to be maintained within .+-.3% in order to avoid loss of production. In prior art processes, the control of the melt index has been carried out by the method in which volumes of ethylene and hydrogen are controlled by means of operation of their respective feed valves in response to the melt index measured. However, this method is deficient because of the delay in the response due to the fact that it takes approximately an hour to accomplish the measurement of the melt index. Therefore, a solution to this problem has been proposed by a method monitoring the inside of the reactor directly, that is, to determine the concentrations of olefin and hydrogen of the liquid phase in the reactor. However, this latter method is deficient for the following reasons:
1. The liquid phase to be measured must be separated from the reaction mixture or slurry composed of liquid and solid.
Under such measurement conditions, it is difficult to obtain the absolute values of olefin and hydrogen concentrations because the components to be measured tend to splash from the liquid phase.
2. A sampling line must be provided with to collect samples from the reactor. Under such sampling conditions, it is difficult to know the exact reaction conditions in the reactor because polymerization occurs before sampling is accomplished (U.S. Pat. No. 3,835,106).
In prior art processes for preparing polyethylenes, control of the density has been also carried out by the method in which the feed volumes of ethylene and comonomer are controlled by means of operation of respective valves in response to the density measured of the formed polyethylene. However, the method also encounters difficulty related to delay in the response due to the fact that is takes approximately an hour to accomplish the measurement of the density. For the solution of this problem, a method to monitor the inside of the reactor directly has also been proposed. The method, however, is not satisfactory for same reasons as described above in connection with the method wherein the melt index is measured. Another prior art method involves monitoring the inside of the reactor and measuring the pressure in the reactor in order to control the properties of the final product or the ethylene copolymer having uniform quality (U.S. Pat. No. 3,691,142). However, as of yet no completely satisfactory method for producing polyolefins having desired properties, in particular, the desired melt index and/or density has previously been devised.
It is, therefore, an object of the invention to provide a process for preparing polyolefins having extremely uniform predetermined properties in the presence of a Ziegler catalyst and hydrogen by polymerizing olefin, which comprises analyzing the gas phase established in the reactor thereby avoiding the disadvantages involved in the measurements of the liquid phase as mentioned above.
It is another object of the invention to provide a process for preparing polyolefins having extremely uniform predetermined melt index in the presence of a Ziegler catalyst and hydrogen by polymerizing olefin, which comprises measuring the gas phase components set up in the reactor.
It is still another object of the invention to provide a process for preparing polyolefins having extremely uniform predetermined density in the presence of a Ziegler catalyst and hydrogen by polymerizing olefin, which comprises measuring the gas phase components set up in the reactor.